Objectives of PPTs

Be able to describe the basic processes of fabrication

Be able to explain the principles of Photolithography.

Be able to describe the basic mechanisms of the additive

processes (Oxidation, PVD & CVD), including relative

comparisons among them.

Physical Vapor Deposition (Evaporation & Sputtering)

Chemical Vapor Deposition

Be able to describe the basic mechanisms of the subtractive

processes (Dry & Wet Etching), including relative

comparisons among them.

Wet Etching (Isotropic & Anisotropic)

Dry Etching (Physical, Chemical, Physical-chemical)

Be able to describe the process of bonding and packaging

Silicon Review In a perfect crystal, each of silicon’s

four outer electrons form covalent

bonds, resulting in poor electron

mobility (i.e. insulating)

Doping silicon with impurities alters

electron mobility (i.e. semiconducting)

Extra electron (“N-type”, with

phosphorous, for example)

Missing electron (“P-type”, with boron,

for example)

Silicon Micromachines

The other application ismicromachines, also called themicroelectric mechanical system(MEMS), which have thepotential of making the computerobsolete.

The micromachines include: Fuel cells

DNA chips

……

Fabrication Silicon crystal structure is regular, well-understood,

and to a large extent controllable.

It is all about control: the size of a transistor is 1 m, thedoping must therefore less than have of that

How to control?

Fabrication Techniques

Fabrication Process of Si Devices

Single crystal

growingWafer

slicing

Film

depositionOxidation

DiffusionIon

implantationEtching Lithography

Metallization Bonding Packaging Testing

Silicon Wafer Fabrication

Crystal Growing

Silicon occurs naturally in the forms

of silicon dioxide and various

silicates and hence, must be purified

The process of purifying silicon:

Heating to produce 95% ~ 98% pure

polycrystalline silicon

Using Czochralski (CZ) process to grow

single crystal silicon

1 rev/s

10 m/s

Liquid

silicon

Illustration of CZ process

Crystal Growing

Czochralski (CZ) Method

Wafer Slicing

This step includes

Slice the ingot into slices

using a diamond saw

Polish the surface, and

Sort

Film Deposits

This step is used to add a special layer on the surface of

the silicon for masking

Many types of films are used for insulating / conducting,

including polysilicon, silicon nitride, silicon dioxide,

tungsten, and titanium.

Films may be deposited using various method,

including

Evaporation

Sputtering

Chemical Vapor Deposition (CVD)

Film Deposits

The process of CVD(a) Continuous, atmospheric-pressure CVD

(b) Low-pressure CVC

Clean wafer

Deposit barrier layer

SiO2, Si3N4, metal

Coat with photoresist

Soft bake

Align masks

Substrate

SiO2

Substrate

SiO2

PR

Substrate

SiO2

PR

Light

Photolithography

Expose pattern

Develop photoresist

Hard bake

Etch windows in barrier layer

Remove

photoresist

Substrate

SiO2

PR

Substrate

PR

SiO2

Substrate

SiO2

Photolithography

Si wafer cleaning procedure

Solvent removal

Removal of residual organic/ionic contamination

Hydrous oxide removal

Heavy metal clean

Photolithography

Barrier layer formation

The most common material: SiO2

Si3N4, polysilicon, photoresist and metals are used

at different points in a process flow

Thermal oxidation, CVD, Sputtering and Vacuum

Evaporation.

PhotolithographyPhotoresist Application:

Surface must be clean and dry for adhesion

A liquid adhesion promoter is often applied

To make 2.5 to 0.5 µm thick layer, 1000 to 5000

RPM for 30 to 60 sec

The actual thickness viscosity

1/(spinning speed)0.5

Photolithography

Photolithography: is a process by which an image is

optically transferred from one surface to another, most

commonly by the projection of light through a mask onto a

photosensitive material (Photoresist material).

Photoresist: is a material that changes molecular structure

when exposed to radiation (e.g. ultraviolet light). It

typically consists of a polymer resin, a radiation sensitizer,

and a carrier solvent.

Photolithography-spin-coatingAdding a photoresist layer on

the wafer

A Photomask is typically

manifested as a glass plate

with a thin metal layer, that is

selectively patterned to define

opaque and transparent

regions.

Photolithography

Photoresist Exposure and Development

The photoresist is exposed through the mask with a

proper light

The photoresist is developed with a developer

supplied by the manufacturer

A positive resist and a negative resist

The positive resist yields better process control in small-

geometry structures

PhotolithographyA positive photoresist is weakened by

radiation exposure, so the remaining

pattern after being subject to a developer

solution looks just like the opaque

regions of the mask

A negative photoresist is strengthened by

radiation exposure, so the remaining

pattern after being subject to a developer

solution appears as the inverse of the

opaque regions of the mask.

Processing Equipments

Wafer aligner and exposure tool:

Photolithography-exposure

Mask alignment:

Square glass plate with a patterned emulsion or metal film is

placed 25 to 125µm over the wafer

With manual alignment, the wafer is held on a vacuum chuck

and carefully moved into position

Computer-controlled alignment equipment achieves high

precision alignment

Alignment marks are introduced to align each new mask level

to one of the previous levels.

UV Exposure: Light Source

High pressure mercury arc lamp → UV

Mercury/Xenon lamp → UV

Excimer laser (KrF, ArF) → DUV (KrF : 248 nm)

Electron beams

X-ray

Exposed Energy Energy(mJ) = Light intensity(mW) * time(s)

Light Spectrum i line : 365 nm

g line : 436 nm

h line : 405 nm

Various Printing Techniques

Lens

Mask

SiO2Photoresist

Wafer Wafer

Wafer

Space

Lens I

Lens II

Mask

SiO2

Photoresist

(a) (b)

(c)

(a) Contact printing, (b) Proximity printing, (c) Projection printing

Ultraviolet LightSource

Photolithography-Baking Soft Baking (Pre-baking)

To improve adhesion & remove solvent from PR

10 to 30min. in an oven at 80 to 90 ºC

Manufacturer’s data sheets

Hard Baking

To harden the PR and improve adhesion to the substrate

20 to 30 min. at 120 to 180 ºC

Manufacturer’s data sheets

Photolithography-EtchingEtching techniques

Wet chemical etching

Dry etching

Plasma, sputter, RIE, CAIBE, ECR

Photoresist removal

Liquid resist strippers cause the resist to swell and lose

adhesion to the substrate

Resist ashing: oxidizing(burning) it in an oxygen plasma

system

Dry Etching MechanismsPhysical Removal based on impact & momentum transfer

Poor material selectivity

Good directional control

High excitation energy

Lower pressure, 100 mTorr

Physical/Chemical

1. Good directional control & 2. Intermediate pressure, ~100 mTorr

Isotropic Wet EtchingEtch occurs in all crystallographic directions at the same

rate.

Most common formulation is mixture of hydrofluoric, nitric

and acetic acids (“HNA”: HF + HNO3 + CH3COOH).

Etch rate may be very fast, many microns per minute.

Masks are undercut.

High aspect ratio difficult because of diffusion limits.

Stirring enhances isotropy.

Isotropic wet etching is

applicable to many materials

besides silicon.

Anisotropic Wet Etching

Etch occurs at different rates depending on exposed crystal

Usually in alkaline solutions (KOH, TMAH).

Heating typically required for rate control (e.g. > 80 oC).

Etch rate typically ~1 µm/min, limited by reactions rather

than diffusion.

Maintains mask boundaries without undercut.

Angles determined by crystal structure (e.g. 54.7º).

Possible to get perfect orthogonal

shapes outlines using 1-0-0 wafers.

Etching –Comparison

ISOTROPIC

Wide variety of materials

No crystal alignment

required

May be very fast

Sometimes less demand

for mask resilience

ANISOTROPIC

⚫Predictable profile

⚫Better depth control

⚫No mask undercutting

⚫Possibility of close feature

arrangement

Multiple layers are common

Dry and Wet Etching: Comparison